LinuxKPI: Rename linux_list_add to __list_add() in linux/list.h

to match Linux

Sponsored by: Serenity Cyber Security, LLC
Reviewed by: manu, bz
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42818

(cherry picked from commit c58ddc25462699c0d13bf703adbed661bb102cac)